Improvements for Dockerfile.

Installs from the local code rather than a release version, creates a non-root user, and switches from entrypoint to cmd for better customization.

Dockerfile

@@ -1,16 +1,19 @@
-FROM python:3.6.6-alpine3.8 as builder
+FROM python:3.6-alpine as builder
 
 RUN apk --no-cache add g++ zeromq-dev
-RUN pip install locustio pyzmq
+COPY . /src
+WORKDIR /src
+RUN pip install .
 
-FROM python:3.6.6-alpine3.8
+FROM python:3.6-alpine
 
-RUN apk --no-cache add zeromq-dev
+RUN apk --no-cache add zeromq && adduser -h / -s /bin/false -H -D locust
 COPY --from=builder /usr/local/lib/python3.6/site-packages /usr/local/lib/python3.6/site-packages
 COPY --from=builder /usr/local/bin/locust /usr/local/bin/locust
 COPY docker_start.sh docker_start.sh
 RUN chmod +x docker_start.sh
 
 EXPOSE 8089 5557 5558
 
-ENTRYPOINT ["./docker_start.sh"]
+USER locust
+CMD ["./docker_start.sh"]